It is well established that speech that is projected or transmitted into a region, area, or space is not necessarily intelligible simply because it is can be heard within the space. It is common for speech projected within particular buildings or structures, such as sports stadiums, airports, or office buildings, to be loud enough to be heard by human listeners yet still be unintelligible. Unintelligibility of audible speech can result from a variety of factors, including, but not limited to, background noise, the quality of an audio system over which the speech is broadcast, and the architecture of a structure into which the speech is projected. For example, a room having a certain shape or configuration can cause excessive reverberation of sound therein, thereby garbling and diminishing the intelligibility of a projected speech message.
The intelligibility of broadcast speech should be taken into consideration in the implementation of audio announcement systems in general, and especially those systems that are associated with fire safety and building monitoring. The intelligibility of such systems are of particular concern because they may be responsible for transmitting warnings, notifications, and instructions relating to situations and conditions that are potentially hazardous to human life.
Over the past several decades, quantitative methods have been developed for testing the intelligibility of announcement systems in a manner that provides a greater degree of reliability and repeatability than previous, subjective methods. These quantitative methods typically employ a standardized Speech Transmission Index (STI) that facilitates evaluation of speech intelligibility without any need for subjective, human interpretation of transmitted speech. STI-type testing is typically administered by specially-trained technicians and involves playing amplitude-modulated noise signals through an audio announcement system that is being tested, such as by projecting the noise signals through one or more loud speakers that are part of the announcement system. The noise signals are detected by a specialized testing device that is carried through regions, areas, or spaces into which the noise signals are projected. The testing device analyzes the test signals by comparing the depth of modulation thereof with that of a substantially identical test signal that is stored locally in the device's memory. Relative reductions in modulation depth of the received signals compared to the stored signals are associated with a loss of intelligibility.
The above-described evaluation process yields a numerical, STI intelligibility score in a range between 0 and 1 with which announcement systems are graded. A score of 0 indicates complete unintelligibility and a score of 1 indicates perfect intelligibility. In order for an audio announcement system to be considered acceptable, the National Fire Protection Association (NFPA) requires that at least 90 percent of the measurement locations within a building have an STI score of not less than 0.45 and an average STI score of not less than 0.50 (NFPA 72, 2010, Annex D).
A problem that is commonly associated with conventional, STI-type intelligibility tests is that the noise signals that are necessarily injected into and played through announcement systems during such testing are extremely intrusive and aggravating to humans. As a result, intelligibility tests are typically only performed in buildings and structures when they are unoccupied, such as after work hours or prior to a tenant moving into a space. This presents a problem, because intelligibility tests that are performed in unoccupied and/or unfurnished structures can yield results that are highly inaccurate relative to actual operating conditions, wherein background noise that is generated by occupants and equipment can have a significant impact on an announcement system's intelligibility. Still worse, in some cases intelligibility testing is often completely forgone in facilities that are continuously occupied, such as hotels and hospitals.